A method of forming a color or monochrome image according to a known prior art is such that an ink sheet containing thermal diffusive dye, which has the property of being diffused and transferred by heat, is placed opposite to an image receiving layer of an image receiving sheet and the thermal diffusive dye is transferred onto the image receiving layer to form an image, using a thermal printing means such as thermal head or laser. The above thermal transfer method has been acknowledged as a method that enables to form an image from digital data and also to form a high quality image comparable with a silver salt picture without using any processing solution such as a developer.
Concerning the storage stability and permanence, however, the quality of an image formed by this method has not yet reached those of a silver salt picture.
In order to improve the stability of a formed image, particularly to improve the fixing stability and light resistance, there have been disclosed thermal transfer materials using a chelatable thermal diffusive dye (hereinafter also called as a post-chelate dye) and methods of forming images (post-chelate technique) in the Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) Nos. 59-78893, 59-109349 and 60-2398 for example.
As a technology of improving the mechanical permanence (e.g., abrasion resistance, grease resistance) of the image formed by a dye thermal transfer method, there has been proposed a technology for forming a transparent protection layer on an image by the thermal transfer method after the image is formed; and a process of using this technology for the image formed by the post-chelate technique has also been disclosed (refer to the Patent Document 1 for example).
When forming an image by a thermal sublimation transfer method, since the thermal transfer sheet and the thermal transfer image receiving sheet are put together and heated while conveyed through a printer, there may arise a problem that static electricity is generated resulting in a trouble in the conveyance or that dust is collected on the dye receiving layer surface of the thermal transfer image receiving sheet resulting in imperfect coloring. In addition, as a method for forming a protection layer described above, there is available a method of forming a protection layer on the formed image using a thermal transfer sheet on which a transferable protection layer has been provided beforehand. While the protection layer described above is transferred by a thermal printer, there has often been a problem that a considerable amount of static electricity is generated when the protection layer is separated from the thermal transfer sheet, resulting in a trouble in the conveyance of a thermal transfer image receiving sheet and thermal transfer sheet in the thermal printer.
In addition, when multiple image prints are piled one over another after printing, the image receiving sheets adhere to each other due to static electricity, and therefore there sometimes arises a problem that multiple image prints cannot be piled up compactly, resulting in inconvenience in handling the prints.
In order to solve the above problems, there have been proposed ideas of eliminating static electricity by providing an antistatic layer on a thermal transfer sheet (ribbon) or impregnating the thermal transfer sheet with antistatic agent in JP-A 9-52454, 7-179071, 7-179072, 6-55868, 6-99670, 10-81078, 10-118565, 10-119444, 8-300842, 9-156244, and 9-295465 for example. Recently, there have also been proposed antistatic techniques (refer to the Patent Documents 2–5 for example). With the above proposed methods, however, electrostatic charge of the thermal transfer image receiving sheet cannot be prevented fully satisfactorily.
There have also been disclosed methods of eliminating static electricity by providing an antistatic layer on the back surface of the thermal transfer image receiving sheet or impregnating the back surface with an antistatic agent in JP-A Nos. 4-366688, 5-58064, 7-1845, 8-175035, 9-207462, 10-35116, 10-44624, 10-58846, 11-157226 and 11-165469 for example. However, these proposed methods of providing a conductive layer on the back surface are not fully enough to prevent generation of static electricity on the image receiving surface.
There have also been disclosed methods of providing a conductive layer on the image receiving surface in JP-A 5-64979, 6-155949, 7-32754, 7-290845, 8-52945, 10-324072, 10-329432, 11-78255, and 11-321125. Even with these technology in which a formed image print contains a transferable protection layer, however, static electricity prevention has not been fully effective enough. With a method in which a protection layer is separated and transferred, compared to a method in which no protection layer is separated and transferred, it is supposed that the intended effect of the technique has not yet been fully produced because a lot of electric charge is generated and also because the conductive layer is destructed by the heat caused in transferring the protection layer.
There have also been disclosed methods of providing a protection layer with an antistatic agent so as to prevent electrostatic charge of the formed image print containing the transferable protection layer described above (refer to the Patent Documents 6 and 7 for example). These proposed methods are effective for electrostatic prevention of the formed image print containing a transferable protection layer, however, involve a problem that the intrinsically intended properties of the protection layer, namely permanence and storage stability of the formed image are not fully attained.
There have been proposed processes of using cellulose resin as a backing layer provided on the image receiving sheet in the thermal transfer method (refer to the Patent Documents 8 and 9). In these patent documents, however, almost no information have been disclosed on the relationship between the antistatic property and the use of cellulose resin nor on the trial to improve the antistatic property by considering the conductivity measured by the salt bridge method.
[Patent Document 1]
JP-A 2001-168244
[Patent Document 2]
JP-A 2000-103175
[Patent Document 3]
JP-A 2000-103178
[Patent Document 4]
JP-A 2000-272254
[Patent Document 5]
JP-A 2001-1653
[Patent Document 6]
JP-A 11-105437
[Patent Document 7]
JP-A 2003-145946
[Patent Document 8]
JP-A 10-297113
[Patent Document 9]
JP-A 11-181226